The present invention relates to a hydrocyclone for mineral separation.
The invention is particularly concerned with the separation of different-sized particles of the same or similar densities i.e., similar specific gravities, and has been developed with a view to improving the separation of china clay.
In the china clay industry, the kaolin particles washed out of the kaolinized matrix are separated into different grades of material for different uses according to particle size, the very finest clay being used, for example, in the paper industry. This separation is carried out in various stages in settling tanks, centrifuges and/or hydrocyclones.
The final separation stage, giving fine kaolin with an extremely low residual content of coarser particles, is usually carried out in settling tanks, comprising enormous concrete structures which are extremely expensive to build and maintain, and the object of the present invention is to provide an improved hydrocyclone separator which is able to achieve comparable results at reduced costs.
As is known, a hydrocyclone comprises a hollow body defining a separating chamber having a cylindrical portion opening into a coaxial frusto-conical portion which tapers to a first axial outlet, the body also having a tangential inlet to the cylindrical chamber portion adjacent an end wall thereof and a hollow spigot projecting coaxially from the end wall into the separating chamber to define a second axial outlet from the chamber, the spigot having an axial extent slightly greater than that of the inlet.
In use, the hydrocyclone is arranged with its axis vertical and the inlet at its upper end. A suspension containing particles of different sizes is fed in through the inlet and enters the chamber around the hollow spigot, termed a vortex finder. By virtue of the configuration of the inlet and of the hydrocyclone generally, the suspension is forced to rotate downwardly and inwardly as the chamber tapers, creating a primary vortex flow adjacent the hydrocyclone wall. Centrifugal forces acting on the particles in the suspension cause larger, heavier particles to be entrained with this primary vortex flow which exits through the lower outlet as the underflow while lighter particles are entrained in a secondary, upwardly-moving vortex flow created in the central part of the hydrocyclone and exit with the flow (overflow) through the second, or upper, outlet. The separation achieved is not, however, complete: a certain proportion of larger particles is entrained with the lighter one and vice versa and a cut point, d.sub.50, is defined for any one hydrocyclone, this being the size of particle which stands an equal chance of exiting with the overflow or the underflow.
The d.sub.50 value for a given hydrocyclone is governed by many factors, the most important of which are the vortex-finder diameter, the feed pulp (suspension) density and the inlet pressure: in general the d.sub.50 value is reduced as the vortex-finder diameter and the pulp density are reduced and the inlet pressure is increased, but reductions in the first two factors also result in reductions in throughput. With a knowledge of these and other factors, hydrocyclones can be designed with appropriate d.sub.50 values for different uses, even down to the fine cut point needed to provide an overflow suitable for paper making, but it has not until now been possible to reduce the proportion of larger particles in the overflow to a desirable extent with commercially-viable flows. It is thus the object of the present invention to improve the performance of hydrocyclones and this has been found to be possible by a most unexpected modification.